(a) Field of the Invention
This invention relates to moldable sulphur compositions and molded articles based on sulphur and mineral aggregate and their method of preparation; more especially the invention is concerned with sulphur concrete, sulphur mortar and other sulphur-bound compositions of improved durability under cyclical freezing and thawing.
(b) Description of the Prior Art
Mixtures of mineral aggretate and sulphur, wherein the sulphur functions as a binder are termed sulphur concretes or sulphur mortars depending on the form of the mineral aggregate.
Articles which may be formed by casting such sulphur concretes include preformed concrete articles conventionally formed from concretes based on a mixture of a hydraulic cement, such as Portland cement, mineral aggregate and water together with various additives. Such articles include paving slabs, structural members, curbing, gutters, pipes and other cast products.
Sulphur mortars may be employed, for example, in the cementing or joining of pipe, bricks and tile, and as coating compositions to provide a protective surface.
Sulphur concretes and mortars display certain advantages, as compared with Portland cement, although they also suffer from certain disadvantages.
Sulphur is available in large quantities both as a by-product of oil refining and natural gas processing in the petroleum industry and from mining of sulphur deposits.
A particular disadvantage of prior sulphur concretes is that they display poor freeze/thaw durability, and thus have poor resistance, and break down, when subjected to repeated cycles of freezing and thawing, in climates such as those encountered in Canada and the Northern States of the U.S.A.
In particular, sulphur concretes and mortars typically have a freeze/thaw life of only up to 80 cycles and frequently only 20 to 40 cycles, whereas a life of at least 300 cycles is the criterion set by ASTM C-666 for acceptable freeze/thaw durability in concretes and mortars to be exposed to the climates of Canada and the Northern States of the U.S.A.
The poor freeze/thaw durability of sulphur concretes is well recognized and has been noted by R. E. Loov--Sulphur Concrete--, State of the Art in 1974, Research Report No. CE75-2, Department of Civil Engineering, The University of Calgary, Alberta, March 1975; V. M. Malhotra, Mineral Properties and Freeze-Thaw Resistance of Sulphur Concrete, Mines Branch Investigation Report 1R 73-18, Department of Energy, Mines and Resources, Ottawa, Ontario, January 1973; J. J. Beaudoin et al., Freeze-Thaw Durability of Sulphur Concrete, Building Research Note No. 92, Division of Building Research, National Research Council, Canada, June 1974; and T. A. Sullivan et al, Development and Testing of Superior Sulfur Concretes, Report of Investigations 8160, Bureau of Mines, United States Department of the Interior, 1976.
In the Portland cement art it is known to entrain air in Portland cement to improve various properties. Such air entrainment is described in U.S. Pat. Nos. 3,615,784; 3,782,983; 4,019,916; and 4,046,582.
The poor freeze-thaw durability of Portland cement concrete has been considered to be due to pressures generated by moisture movement through pores inherent in the structure of such cement. The migration of the water in freeze/thaw cycles results in pressure build-up within the Portland cement structure, and the pressure is relieved by formations of fissures or fractures in the cement, with consequent failure of the cement and concrete.
Formation of air voids and entrainment of air in Portland cement has been utilized to provide spaces or air pockets throughout the Portland cement structure, into which the water inherent in the Portland cement structure can expand during freeze/thaw cycles. In this way pressure build-up is avoided or reduced so that the incidence of fissures or fractures is reduced.
Water is not inherent in sulphur concrete and sulphur mortar structures, and the poor freeze/thaw durability of such sulphur compositions has been considered by the art to be due to expansion and contraction of sulphur crystals in the structure. Sulphur has an unusually high coefficient of thermal expansion (.alpha..apprxeq.55.times.10.sup.-6 /.degree.C.) and a low thermal conductivity (0.27 W/MK). Temperature gradients in sulphur concrete thus generate high levels of differential stress, and the disintegration or destruction of sulphur concretes under conditions of freezing and thawing has been attributed to this. Efforts to improve the freeze/thaw durability have centered on plasticizing the sulphur and producing a non-crystalline amorphous or polymerized structure or one in which the sulphur crystals are of small size. Characteristics of sulphur cements resulting from dimensional changes in the crystals or from changes in the crystal structure with fluctuating temperature and the use of agents, such as olefine polysulfides, to modify the crystalline habits of sulphur have been described by W. W. Duecker, Admixtures Improve Properties of Sulphur Cements, Chemical & Metallurgical Engineering, Nov. 1934, pp. 583-61.
The need to modify sulphur, as by plasticization to overcome inferior freeze/thaw durability characteristics has also been described by B. R. Currel, New Sulphur-based Coatings, Polymers Paint and Colour Journal, 1978, pages 674-8; T. A. Sullivan and W. C. McBee, Sulphur Institute Journal, Spring 1976, pages 6-8; T. A. Sullivan and W. C. McBee, Development of Specialized Sulphur Concrete, Proc. Int. Conf. on Sulphur in Construction, Ottawa, 1978, Vol. 2, pages 453-474; Sulphur Research and Development Vol. 2, 1979, pages 2-17; V. M. Malhotra, Sulphur Concrete and Sulphur Infiltrated Concrete: Properties, Applications and Limitations, Canmet Report 79-28, Energy, Mines and Resources Canada, May 1979, pages (i) and 2-11; and U.S. Pat. No. 4,058,500 Vroom. In particular Sullivan and McBee and Malhotra refer to the need for modification as by plasticization, because of the transformation of molten sulphur, on cooling below 95.5.degree. C., from the monoclinic form to the orthorhombic crystalline form which is denser, occupies less volume and is subject to disintegration on thermal cycling.
Thus, it has generally been accepted by persons in the art that the poor durability displayed by sulphur concretes and similar sulphur/mineral aggregate compositions, subjected to fluctuating temperature conditions, such as in freeze/thaw cycles, is due to changes in the crystals of the sulphur matrix with temperature. In particular sulphur crystals expand with increase in temperature and contract with decrease in temperature; in addition the molecular structure may change between its various crystalline and non-crystalline forms.
The contractions and expansions experienced in freeze/thaw cycles produce different stresses in the composition, both in the sulphur matrix and between the sulphur matrix and the particles of the aggregate. In particular the volume expansion of the crystals with increase in temperature and/or morphological changes, and the resulting stress produced, was generally accepted as being responsible for the development of fissures or fractures and the consequent deterioration and failure of the composition.
It should be recognized that in the Northern climates of the North American Continent freeze/thaw cycles may occur frequently over short periods of time, with, for example, freezing occurring overnight and thawing during the daylight hours, on a daily basis.
As indicated above by reference to Duecker and others, attempts to overcome the problem of poor freeze/thaw durability in such sulphur compositions have thus focussed on modifying the structure assumed by the sulphur in the solid state.
Sulphur concretes and mortars containing entrained air have not previously been proposed, although foamed sulphur has been described by Dale and Ludwig in U.S. Pat. Nos. 3,337,355 and 4,022,857; the foams of Dale and Ludwig have densities below 30 lbs/cu.ft. Dale and Ludwig found it necessary to employ a viscosity increaser in conjunction with a stabilizing agent in the molten sulphur; the viscosity increaser was considered necessary to retain the gas bubbles from the foaming agent in the molten sulphur, and the stabilizing agent was considered necessary to prevent coalescence of the bubbles.
It is an object of this invention to provide compositions based on sulphur and a mineral aggregate and in particular hot moldable sulphur concretes and mortars, which are freeze/thaw durable.
It is a further object of this invention to provide molded sulphur concrete articles of good freeze/thaw durability.
It is a still further object of this invention to provide a method of producing such compositions and articles.